1. Field of the Invention
The present invention relates to a method of preparing inorganic and/or organic surfaces comprising organized micro- or nanostructures, to the micro- or nanostructured surfaces obtained by application of this method, and to the various applications of these structured surfaces, notably in the area of photonics, catalysis, magnetic storage or biosensors.
2. Description of Related Art
Methods of preparing nanostructured surfaces, i.e. more generally surfaces comprising organic or inorganic nanostructures, are the object of constant research, as these surfaces have varied applications depending on the nature and morphology of they nanostructures.
Surfaces covered with organized metallic nanostructures find applications in emerging fields such as nanophotonics, or as a substrate for surface-enhanced Raman spectroscopy (SERS). One of the properties of these surfaces is to increase the Raman signal obtained by several orders of magnitude, hence their interest in the analytical and bio-analytical industry for biosensors. In fact, when metallic structures are sufficiently close together, their surface plasmons can then be coupled, generating electromagnetic “hot spots” responsible for the signal enhancement effect. The distance between these structures is therefore a crucial parameter requiring fine control.
Arrays of polymer nanostructures are designed essentially for biotechnology applications, the nanostructures serving as anchoring points for biomolecules such as oligonucleotides for DNA chips, or proteins, notably enzymes, for biosensors. One of the difficulties is that these nanostructures must be separated by a matrix, on which the biomolecules that are to be anchored specifically on the nanostructures cannot become attached. When the polymer used for fabricating the nanostructures is an electrically conducting polymer, such as a polypyrrole for example, the arrays can then have applications as micro-/nano-electrodes.
Nanostructured surfaces can basically be prepared according to two broad types of techniques: “sequential” techniques and “parallel” or “masking” techniques.
According to the “sequential” technique, the nanostructures are deposited on the surface of a substrate point-by-point by scanning the surface with sophisticated apparatus of the microscope tip type (electron or atomic force or tunnel-effect) or with an ion or electron beam. These techniques are expensive, time-consuming and require considerable know-how. Conversely, the sizes obtained are small, of the order of a few nanometers.
According to the “parallel” technique, deposition is carried out on a surface that has been masked beforehand. The masks most widely used are of porous alumina as the size of the pores and their spacing can be controlled during manufacture. Nevertheless, this method is still time-consuming as it requires two steps for synthesis of the mask, as well as several steps for deposition of the metal, for example by electron beam evaporation, vacuum evaporation or else electrolytically, then a step of removal of the mask to obtain the anticipated array of nanodots. Moreover, this method has risks of notable chemical contamination.
An alternative to using these porous masks is to use colloidal lithography. For example, notably in the article of Bayati M. et al., Langmuir, 2010, 26(5), 3549-3554, a method has already been proposed for fabricating surfaces comprising metal (gold, platinum, copper) nanorings, consisting of covering the surface of a substrate, on which deposition is to be effected, with a dispersion of polystyrene beads, which will self-organize in a monolayer on the surface of said substrate, then, after evaporation of the solvent (water), covering the substrate with a solution of a metal precursor (metal salt) so that the precursor infiltrates, by capillarity, the spaces left free between the polystyrene beads. After reduction of the metal salt to cause fixation of the metal on the surface, the substrate is rinsed, then the polystyrene beads constituting the mask are removed by treatment with chloroform. A metal deposit is thus obtained that is either in the form of nanodots if the time of impregnation with the solution of metal salt was long, or in the form of nanorings if the time of impregnation with the solution of metal salt was short. However, this method is time-consuming in application and does not allow the morphology and size of the deposits obtained to be modulated with high precision.
The use of masks consisting of an array of colloidal particles has also been envisaged for preparing nanostructured surfaces with an array of polymer dots. Thus, Valsevia et al. (Adv. Func. Mat., 2006, 16, 1242-1246) propose for example a method of surface nanostructuring consisting of depositing a layer of polymer (polyacrylic acid: PAA) by the technique of plasma-enhanced deposition on a substrate, then depositing, by spin-coating on said layer of polymer, a layer of colloidal spheres in the form of a compact hexagonal array. The method then comprises a step during which these spheres are abraded under plasma in order to reduce their size, then an additional step makes it possible to deposit another polymer (polyethylene glycol: PEG) on these spheres and on the layer of PAA that has been made accessible by abrasion of the colloidal spheres. Finally, the spheres are removed, revealing PAA dots organized in a hexagonal array in a matrix of PEG According to this method, the size of the PAA dots is controlled by the size of the spheres and the abrasion time, parameters which also allow control of the distance between the dots. This method is therefore complex and requires multiple steps, as well as expensive equipment.
Currently, no method exists that allows nanostructured surfaces with controlled morphology, thickness and roughness to be obtained simply, in an easily modulated manner, inexpensively and in a minimum of steps.